1. Field of the Invention
This invention relates to antenna structures and more particularly to a novel and highly effective antenna structure comprising a radiating element such as a patch antenna in combination with a ground plane constructed to enhance antenna performance.
2. Description of the Prior Art
There is a need for an improved antenna structure for use with a GPS receiver that receives and processes signals from navigation satellites. Antenna structures known heretofore that are capable of optimum performance are too bulky and unwieldy for use in small GPS receivers, especially hand-held receivers. Compact antenna structures that are conventionally employed with GPS receivers do not provide optimum performance. One problem is that they receive signals directly from satellites and, because of ground reflections, also indirectly. This so-called multipath reception causes time measurement errors that can lead to a geographical fix that is erroneous or at least suspect.
A British patent publication No. 2,057,773 of Marconi discloses a large radio transmitting antenna including aerial wires supported in spaced, parallel relation by posts. The ground around the antenna is saturated to a depth of two or three meters with an aqueous solution of calcium sulfate to increase the conductivity of the ground and thereby improve its reflectivity. The ground is permeated to a distance two to three times as far from the antenna as the antenna is tall. In a typical case this can be from 50 to 100 meters from the boundaries of the antenna array.
A European patent publication No. 394,960 of Kokusai Denshin Denwa discloses a microstrip antenna having a radiation conductor and a ground conductor on opposite sides of a dielectric substrate. The spacing between the radiation conductor and the ground conductor, or the thickness of the dielectric substrate, is larger at the peripheral portion of those conductors than at the central portion. Because of the large spacing at the peripheral portion, the impedance at the peripheral portion where electromagnetic waves are radiated is said to be close to the free-space impedance.
A German patent publication No. DE 37 38 513 and its U.S. counterpart patent No. 5,061,938 to Zahn et al. disclose a microstrip antenna including an electrically conductive base plate carrying an electrically insulating substrate on top of which are a plurality of radiating patches. A relatively large spacing is established between the electrically insulating substrate and the base plate at lateral dimensions somewhat larger than lateral dimensions of the patches and also in the vicinity of the patches. The patches and spacings are vertically aligned through either local elevations of the insulating substrate or local indentations in the base plate. The feeder line is thus relatively close to the conductive base plate, and the radiating patch is farther away from the conductive base plate. This is said to improve the radiating characteristics of the patch.
A German patent publication No. DE 43 26 117 of Fischer discloses a cordless telephone with an improved antenna.
A European patent publication No. 318,873 of Toppan Printing Co., Ltd., and Seiko Instruments Inc. discloses an electromagnetic-wave-absorbing element comprising an elongate rectangular body of dielectric material having a bottom portion attachable to an inner wall of an electromagnetically dark room, and peripheral elongate faces extending vertically from the bottom portion. A set of the absorbing elements can be arranged in rows and columns on the wall. An electroconductive ink film is formed on the peripheral faces of the body and has a gradually changing surface resistivity decreasing exponentially lengthwise of the peripheral face toward the bottom portion. The incident electromagnetic wave normal to the wall provided with the rows and columns of absorbing elements is absorbed by a lattice of the electroconductive film during the travel along the electroconductive film. In order to avoid reflection of an incident electromagnetic wave at the boundary between the surrounding air and the absorbing element, the characteristic impedance at the top of the element through which the incident wave enters is close to the impedance of air. In order to avoid reflection at the boundary between the bottom of the element and the wall to which it is attached, the characteristic impedance at the bottom is close to that of the wall. The absorbing element is made of a plastic body with an electroconductive covering and having a variable resistivity or conductivity.
The following prior art is also of interest: U.S. Pat. Nos. 5,592,174 to Nelson for GPS Multi-Path Signal Reception, Raguenet U.S. Pat. No. 5,248,980 for Spacecraft Payload Architecture, Franchi et al. U.S. Pat. No. 5,204,685 for ARC Range Test Facility, Kobus et al. U.S. Pat. No. 5,170,175 for Thin Film Resistive Loading for Antennas, De et al. U.S. Pat. No. 5,132,623 for Method and Apparatus for Broadband Measurement of Dielectric Properties, Hong et al. U.S. Pat. No. 4,965,603 for Optical Beamforming Network for Controlling an RF Phased Array, Schoen U.S. Pat. No. 4,927,251 for Single Pass Phase Conjugate Aberration Correcting Imaging Telescope, and Bhartia et al. U.S. Pat. No. 4,529,987 for Broadband Micropstrip Antennas with Varactor Diodes.
The prior art as exemplified by the patents discussed above does not disclose or suggest an ideal antenna structure for use in a GPS receiver that receives and processes signals from navigation satellites. What is needed in such an environment is an antenna structure that is very light and portable and adapted to hand-held units of the type used, for example, by surveyors.